This application claims the priority of German Patent Application No. 197 55 116.5, filed Dec. 11, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a fuel cell system comprising a proton-conducting electrolyte membrane (i.e., proton exchange membrane) and a method for operating the proton exchange membrane (PEM) fuel cell system.
In EP 0 629 013 B1 and DE 43 18 818 A1, a fuel cell system is described in which the fuel cell is supplied with process air by a compressor driven by an electric motor. To recover the energy contained in the exhaust air from the fuel cell, the compressor is also coupled with an expander. For this purpose, the compressor and expander are mounted on the same shaft. A fuel cell system according to DE 40 21 097 A1 is similarly structured, in which the exhaust air from the fuel cell is conducted to an expansion turbine. The expansion turbine is coupled with a fresh air compressor for supplying air to the fuel cell.
In known fuel cell systems, a catalytic burner is provided as a heat source to which fuel is supplied in the form of (1) moist anode offgas (H.sub.2 and CO.sub.2) from the fuel cell, and (2) methanol.
When compressing process air to the usual working pressure of 3 bars for example, a portion (e.g., approximately 20%) of the power developed by the fuel cell is required for compression. When energy recovery is used in the expander, this percentage drops to about 10 to 15%. Thus, the air supply of the fuel cell still contributes significantly to reducing the efficiency of the system.
At the same time, in order to lower costs and volume, there is a need for higher working pressures on the cathode side (i.e., air side) in order to build a smaller fuel cell having narrower gas channels and to achieve a higher area-related power yield in the fuel cell.
A method of operating a fuel cell system is described in DE 44 46 841 A1 in which both the anode offgas and the exhaust air from the fuel cell are fed to the catalytic burner.
A system for combined generation of electrical and mechanical energy is disclosed in DE 40 32 993 C1. The combustion gases generated in a burner supplied with a gas containing oxygen and a gas containing hydrogen are used to recover mechanical energy in a gas turbine connected downstream. The mechanical energy generated is partly used to drive a compressor to compress the gas containing oxygen that is supplied to the burner. The system also comprises a fuel cell for generating electrical energy whose anode offgas containing hydrogen is fed to the burner.
The goal of the present invention is to provide a PEM fuel cell system and a method for operating a PEM fuel cell system with a high efficiency.
According to the present invention, the expander power can be increased significantly if the expander is supplied with a higher air mass flow; a higher temperature; and possibly a higher pressure. Increasing the air mass flow and the temperature is accomplished according to the present invention by (1) initially supplying the air at the cathode outlet of the PEM fuel cell as the air supply to the catalytic burner before expansion; and (2) operating the expander with the exhaust air from the catalytic burner. The cathode offgas is depleted of oxygen by the fuel cell reaction, but still contains sufficient quantities of oxygen for the reaction in the catalytic burner. With this arrangement, the mass flow guided to the expander is elevated by comparison with known devices because fuel (usually moist anode offgas and methanol) is supplied to the catalytic burner, whose reaction products thus pass into the expander. At the same time, the entire air mass flow supplied to the expander is raised from the fuel cell temperature (typical temperature for PEM fuel cells is approximately 80.degree. C.) to the working temperature of the catalytic burner (e.g., approximately 350.degree. C.). With the mass flow thus heated, a suitable expander designed for these temperatures can be operated directly.
In one advantageous embodiment, this mass flow can also be utilized for preheating the cathode offgas before it enters the catalytic burner.
As a result of the additional energy input to the expander in the form of heat and mass flow, its performance is increased to the point where the compressor drive (e.g., electric motor and rectifier) can be made much smaller. With an optimum adjustment of the pressure level and/or additional supply of combustion gas in the catalytic burner, the compressor drive can even be eliminated entirely.
In PEM fuel cell systems in which the combustion gas for the fuel cells is generated by a high-pressure gas-generating system (for example, in methanol reformation), a further opportunity is provided for increasing the expander power and/or lowering the compressor power, in which the pressure level is increased by means of a second compressor stage upstream from the cathode input of the PEM fuel cell. The second compressor stage is operated by expansion of the gas that is present in the high-pressure gas-generating system, with the gas pressure dropping from the system pressure of the high-pressure gas-generating system (greater than 15 bars, especially between 20 and 30 bars) to the working pressure of the catalytic burner (approximately 3 bars). The expanded gas, after passing through the second compressor stage, is supplied to the catalytic burner thus increasing the mass flow supplied to the expander of the first compressor stage. The second compressor stage can be designed in the form of a compressor coupled with an expander without a motor or a turbocharger.
With this design, not only is the electrical compressor power reduced through increased system efficiency and smaller size, but the efficiency and size of the fuel cell are also positively influenced as well.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.